To evaluate the genetic hazard of chemicals in our environment and understand their mode of action it is necessary to place chemical mutagenesis on a quantitative basis in a eukaryote organism that can be thoroughly and economically analyzed genetically. Ethyl methanesulfonate (EMS) was selected as the mutagen for developing a model test system because of its known mutagenic effectiveness and its simple chemical structure which makes it relatively easy to synthesize with radionuclides having high specific activity. The mutagenic action of EMS is considered due to its alkylation of DNA, a point of view supported by previous work on this project using Drosophila in a bio-assay of blood plasma from hamsters that had been previously injected with EMS. To place the dosage of EMS on a quantitative basis total alkylation of DNA in Drosophila melanogaster spermatozoa is measured on a per sperm cell basis and then further divided into alkylation at specific sites such as the 7 position of guanine. This dosimetry will then permit the construction of dosage response curves with the dose in terms of alkylation and the response in terms of the extensive mutational spectrum developed with Drosophila. This project differs from previous work with alkylation in that we are studying both alkylation and the genetic consequence within the same germinal tissue of a eukaryote. Results of these tests would eventually enable us to determine whether, as the dose is reduced, repair mechanisms become sufficiently effective to reduce the mutational effect to no greater than the spontaneous rate or if there is a genetic hazard due to low level exposure to alkylating agents. This answer is basic to determining the genetic hazard of a constant low level mutagen in our environment. This work is being expanded to include the bisulfite ion, heavy metals and other environmental mutagens.